Table 5.9 Wind Speed and Planetary Surface Conditions for Terrestrial Planets1566,2066,2087

We’ve already hinted at the effects of evolutionary history on a planet’s surface temperature. What else can be said about the overall climate? First of all, the thinner the atmosphere the greater will be the diurnal variations in temperature. This is because a dense, massive atmosphere has more ”thermal inertia.” Since huge amounts of heat are stored, a brief nighttime cooling-off period has very little effect. But if the air is thin and lightweight (as on Mars), very little heat is reposited. Thus, on the night side the surface and the air above it cool rapidly, leading to large swings in temperature between the two sides of the planet. This results in faster-moving winds (Table 5.9), but because the air is less dense the energy available is actually less.

Perhaps one of the most decisive factors in planetary meteorology is the rotation rate of the planet. On a planet such as Venus, where a single “day” lasts months, surface winds are believed to be no more than a few kilometers per hour, maximum.^1257,2041 On worlds with intermediate rotation rates like Earth and Mars, typical wind speeds range around 50-70 km/hour.^1257,2067 Fast-spinning bodies like Jupiter are known to have winds averaging 140-290 km/hour and higher near the equator.^{1141,1257,2045} Naturally, faster rotation and stronger winds means larger Coriolis forces, along with more violent cyclonic disturbances such as tornadoes, hurricanes, typhoons and water-spouts. Also, slow worlds tend to have greater day/night thermal differentials than faster ones because the air is not as well stirred, Surface temperatures are less uniform as a result.²¹⁴

The heat capacity of the molecules in the atmosphere is also important. This may be thought of as the amount of energy which must be added to a unit of air to raise its temperature a fixed amount. It can also be conceptualized in terms of energy loss: How much heat must be lost to drop the atmospheric temperature one degree? 

An atmosphere like Earth’s in every respect but comprised of hydrogen would have nearly fifteen times the heat capacity of normal air. It would thus take fifteen tithes longer to heat up or cool down, so surface temperatures on a hydrogen-atmosphere planet should be pretty much the same every where.¹²⁵⁷ There would be little if any “climate” as we know it on such a terrestrial.¹²⁵⁷

The presence of oceans affects the climate in many ways. Largely pelagic worlds should experience smaller variations in surface temperature because the water acts as a giant thermal buffer.²⁸⁶ On dry worlds, the climate is likely to be more “continental,” or desert-like.²¹⁴ With no seas, meteorology becomes more volatile — weather changes more rapidly.

Many other factors are important too. The winds are driven by the energy supplied fun a planet’s star. Worlds near the inside edge of the habitable zone should therefore have more violent weather, because more energy is available. Unfortunately, life is more complicated than this because of the vagaries of atmospheric evolution, albedo differences, and the problem of self-heating planets (like Jupiter and Saturn).

Another factor which is extremely complicated is the effect of planetary mass and surface gravity on wind and air pressure patterns. If Dole’s empirical relation between mass and angular momentum holds up,* then it is a fair guess that worlds with high mass will have higher velocity winds, in general. And there are other, more subtle problems. For instance, the winds on Mars often blow at more than half the local speed of sound. One wonders what a “transsonic meteorology” might be like.²⁰³⁷

* Using our own solar system as his source of data, Dole finds that angular velocity is directly proportional to the square root of planetary mass for planets which are not tidally braked or locked.²¹⁴

Figure 5.8 Different Patterns of Cyclonic Meteorology

Some insight into comparative meteorology can perhaps be gained by looking at the peculiar manifestations of weather on other planets in the solar system. Mars has global-scale storms the likes of which have never been seen on Earth. Most every Martian year, dust storms enshroud the entire world in a dull-ochre blanket for months on end. Winds exceed 320 km/hr during this time — far in excess of most Earthly hurricanes. Yet Mars has roughly the same rotation rate as our planet, is colder and farther from Sol, and has a thinner and less massive atmosphere. How can such a magnificent storm develop?

A small, natural cyclonic disturbance is where it all begins (Figure 5.8). Airborne particles absorb more sunlight and heat up the surrounding gas; outside of this local turbulence the air is cooler. The temperature differential causes major winds to begin to circulate. While hurricanes on Earth are caused by water vapor condensation near the eye, Martian hurricanes get their energy directly from the sun.²⁰⁴⁴

Earth has a relatively massive atmosphere with large thermal inertia, so temperature changes occur only very slowly. Our planet thus has a long “response time” to change. Not so on Mars. The Martian air responds to changes in temperature in a matter of hours, because its thermal inertia is low. Winds can build up much faster.

The cyclonic disturbance grows larger and the winds go higher still. One planetologist has estimated that once the turbulence extends about ten kilometers vertically and perhaps 50-90 kilometers horizontally, the storm cannot be stopped,¹³¹³A kind of “runaway weather,” the Martian hurricane continues to grow until it virtually covers the globe. At this point, the thermal gradient which drives the winds lessens and finally disappears, and the storm soon begins to taper off.*

Science fiction writer Arthur C. Clarke has considered an unusual form of weather that might exist on cold terrestrials (like Titan), which are thought to possess large amounts of solid ammonia and gaseous methane. We know that the smaller the liquidity range of a thalassogen, the more volatile will be the meteorology. Sudden weather changes should be commonplace. As an example, liquid methane may be present in small pools on Titan in local cold spots on the surface. Because it has such a narrow liquidity range, the methane could abruptly flash into steam at the first gust of warmer air or if there is a momentary break in the clouds. The high winds thus generated, Clarke suggests, might be called “methane monsoons.”¹⁹⁴⁷

Another hard science fictioneer, Hal Clement, has written of the peculiar behavior of weather on planets with very high surface pressures. Gases — and air — are generally at least a thousand times less dense than liquids. But what if we have an atmosphere with a base pressure from 100-1000 times Earth-normal? The air will take on liquid-like densities, becoming thick and viscous.¹⁹³⁶

What can we say about the presence of frozen thalassogen on the planetary surface? It is well-known that for the greater part of its history, Earth was without polar icecaps. We have them now only because we are in the middle of an Ice Age. Ice Ages are believed by some to occur cyclically every 200 million years or so, triggered by small changes in Sol’s output or by orbital and rotational resonances.^2068,3678

(Of course, icecaps need not form only at the poles. A tidally-locked, one-face planet might have a single icecap on the night side only. Or, peculiar resonances between planetary rotation rate and orbital eccentricity could give rise to icecaps located on either side of the equator — although this remains a strictly speculative possibility.²⁰⁷⁰)

Will all planets with open oceans have icebergs? The answer to this deceptively simple question actually has deep climatic significance. We know that the present climate of our world is in a state of very delicate balance. Surface conditions are largely dictated by the overall energy balance. The greenhouse effect acts to hold heat in and trap energy; Earth’s shiny polar caps tend toward the opposite extreme, reflecting energy back into space and cooling the planet.

* Because the Martian atmosphere is only 1% as dense as that of Earth, the wind packs only about 10% as much punch, An astronaut standing in a 320 km/hour gale on the surface of the red planet would feel the equivalent of a 32 km/hour wind on Earth.¹³¹³

Baroclinic flow:

• Climate powered by large temperature differential between equator and poles (ΔT > 10-100°C).
• Vertical pressure gradient minimal.

Characteristic of:

• Planets with low pressures.
• Planets with slower rotation.
• Planets with negligible internal heating, or which are heated from above (e.g. an optically thick atmosphere).
• Planets whose atmospheric constituents have relatively low heat capacity (e.g. O₂, N₂).
• Planets having a solid surface.

CALMS are regions of “Coriolis pileup.” unstable with little wind, source of cyclonic disturbances (hurricanes). Cold, dry air falls, removes low altitude moisture, creating most of world’s deserts.

DOLDRUMS — moist, warm, rising air causes cloud cover “zone” at Equator ± 10° latitude.

PRESSURE REGIONS (low and high) form into localized eddies and whorls.

FEATURES persist for weeks (Earth) or for months (Mars).

Typical examples in our solar system:

• EARTH, MARS (especially in Martian autumn and spring)
• VENUS (single Hadley cell, “symmetric” regime circulation)

Barotropic Flow:

• Climate powered by vertical pressure gradient forces.
• Temperature differential between equator and pole minimal (ΔT < 5°C).

Characteristic of:

• Planets with high pressures.
• Planets with fast rotation.
• Planets with significant internal heating.
• Planets whose atmospheric constituents have relatively large heat capacity (e.g. H₂, He).
• Planets with no solid surface.

ZONES contain moist, warm, rising air.

BELTS contain dry, cool, falling air.

WINDS flow around planet at zone/belt boundaries.

Low and high PRESSURE REGIONS girdle planet in a series of concentric zonal systems.

Atmospheric FEATURES can persist for centuries because there is no solid surface below the weather, and therefore any real frictional drag.

Typical example in our solar system:

• JUPITER
• SATURN

Icebergs are floating chunks of frozen thalassogen. This proves to be a destabilizing factor in Earth’s climate, because ice reflects energy away far better than the liquid water of the oceans. If there is a prolonged, unusually cold spell planetwide and abnormally great: amounts of ice are produced, more of Sol’s life-giving warmth is cast away by the highly reflective ice floating on the surface. Our planet cools because less heat is available. The icecaps spread, and Earth cools still further. The effect is the exact opposite of the runaway greenhouse discussed earlier, and might properly be termed “runaway icecaps” — an Ice Age.

On the other hand, if the solid form of the thalassogen is less reflective (i.e. darker) than the liquid, the climate should be relatively stable. Any ice formed during a sudden cold snap must subsequently absorb more energy than the surrounding liquid — and soon melt. Icecaps would be unlikely, Ice Ages practically impossible.

Similarly, if a thalassogen cannot form floating icebergs, then even if the ice is highly reflective it still will submerge below the surface of the liquid before it can give rise to thermal instability and runaway icecaps. That is, it moves itself out of the way before it can do much damage. Of course, one man’s bread is another’s poison. The lack of icebergs may promote a more stable climate, but it will also make biology much less likely.

If there are no icebergs, and frozen thalassogen sinks to the ocean bottom because it’s denser, then the sea may freeze from the bottom up and thaw only from the top down. Over the normal range of temperature variations, it is entirely possible that the whole body of liquid could freeze solid for various lengths of time. This is xenologically significant, as the viability of life in such an inimical environment must necessarily be greatly decreased.^47,1551

Water is virtually unique in this respect: The frozen form, water-ice, floats atop the liquid form. Water expands slightly when it freezes, so the ice is less dense than the fluid. (Only elemental bismuth metal and a very few other rare substances display this behavior.) Hence, where water is the thalassogen, bergs will float and life is not precluded by the threat of a planetwide oceanic freezeup during cold spells.⁹⁷⁴ (The price paid for this advantage is climatic instability — it would appear that Ice Ages are possible only if water is the thalassogen.)

Table 5.10 Densities of Some Thalassogens of Interest2062,2063,2069

Not so with all other thalassogens of interest. As we see from Table 5.10, no other single thalassogen has the unique property of floating iceberg production. Even if we allow for a dual thalassogen system, say of ammonia and methane,¹⁹⁴⁷ it is rather difficult to arrange for icebergs or floes of solid ice. Ammonia-ice not only sinks in liquid ammonia, but in liquid methane as well.*

But there are a few possibilities. Water icebergs should float on oceans of liquid oxygen, as should methane and ammonia bergs. Water-ice will also float on carbon dioxide seas at the right pressures. But sulfur, hydrogen, carbon dioxide and oxygen floes are probably out of the question on any kind of reasonable plet.

Many other specific meteorological phenomena are also of major interest to xenologists. For instance, clouds and fogs should be common in any atmosphere with reasonable pressures. Condensation nuclei will always be plentiful, and most thalassogens can condense to tiny droplets around them at moderate temperatures. Rain should likewise be a regular occurrence at the surface of worlds possessing large open bodies of liquid thalassogen. (Of course, other things may rain down — such as the periodic volcanic ash “rains” in Iceland.)

The height at which clouds form is a function of humidity, thalassogen vapor pressure, atmospheric thermal lapse rate, and a score of other interrelated factors. The suggestion that more massive worlds with higher gravity must have lower-hanging clouds²⁰⁷⁵ is simply too facile to be of much use to us.

Any planet which has clouds, rain, and sunlight reaching the surface will also have rainbows from time to time. These beautiful spectral arcs are the result of thalassogen droplets suspended in the air, acting as tiny prisms in concert to separate the incoming light into its constituent colors. The larger the droplets, the more intensely vivid the bow will appear.²¹⁴⁹

Ignoring for the moment many other important factors, a larger planet with higher surface gravity will pull raindrops down before they have a chance to grow very large. Rainbows on larger worlds should tend to be rather dim, unimpressive affairs. On smaller worlds, where droplets can grow to larger sizes because they fall more slowly, rainbows should be impressive riots of color.²⁰⁵⁹ Furthermore, if there happens to be a very bright moon overhead or more than one sun; bows might appear in several parts of the sky at the same time.²⁰⁵⁹

* It should be noted that there are some six different allotropic forms of water-ice which form at various temperatures and pressures. Only one of these — ”natural ice” or ice I as the chemists call it — is lighter than water. Ice II through ice VII all sink if placed on the liquid.

How about lightning discharges? Electrical storms occur because molecules are split apart in the upper atmosphere to ions, which are then carried to the ground by dust and rain. This charges up the planet to at least half a million volts from ground to top of atmosphere — a process likely on any world, save for the exact details of scale height and voltage. Planets with regular and intense sand or dust storms may generate intense electrical fields that could lead to more severe or more frequent discharges.¹²³²

Another important factor is the breakdown voltage of the air — the voltage at which a spark will jump a gap of unit distance. A charged cloud may be 100 million volts higher than the surface below, which is high enough for the “spark” to leap to earth. The spark gap voltage for dry air (at 1 atm) is usually listed as 11,000 volts/cm, and can be corrected for variations in temperature, pressure, and humidity. Now, if the atmosphere was comprised of a more conductive gas, such as neon, the spark gap voltage would only be 800 volts/cm (at 1 atm). This means that lightning should occur more frequently in neon (hydrogen, helium, etc.) than in oxygen (nitrogen, halogens, etc.).

This prediction may perhaps claim some support from the radio observations of Jupiter in the last decade or so. Decameter radio wave outbursts lasting from seconds to hours have been detected, with an equivalent energy of trillions of terrestrial lightning strokes per event.⁶⁰⁹ Similar outbursts have been observed on Saturn.²⁰⁹⁷

Will alien worlds have auroras too? Probably. These displays appear at the north and south planetary magnetic poles, and are caused by the funneling of solar wind ions in the converging magnetic field of the planet. Rapidly rotating, massive worlds should tend to have stronger magnetic fields. Also, hotter stars most likely have more vigorous solar winds. We would guess that a 4 M_earth planet with a ten-hour day circling an F5 sun will probably have far more striking auroral displays than a tidally-locked 0.4 M_earth planet orbiting a K2 star.

Mirage physics is also rather interesting. On Earth, mirages often result when there is a layer of warm air lying close to the ground. This air, being hotter, is less dense. It acts as a giant lens. Light coming from the sky near the horizon swoops down close to the ground and is refracted back up.²⁰⁷³ The mirage of water on an open highway is just a smeared-out image of blue sky.

Mirages on Earth generally appear about 100 meters away from the observer at ground level. On Mars, where the atmosphere is so thin the air is hardly heated by the ground at all, the refraction layer is thinner.⁹⁵⁰ The mirage backs away, out to about one kilometer. (To date, no Martian mirages have been photographed by the Viking landers, possibly due to the extreme roughness of the terrain and because the camera horizon is too close.²⁰⁹⁴)

On planets with very high density air, as on Venus, the mirage concept literally takes on new meaning. The transfer of heat from ground to near-surface air is complete, and it is believed by many that the extreme refraction near the ground will cause a kind of “fishbowl effect.”^15,2060 The horizon would appear above the observer at all times,* appearing to bend upward at the sides.²⁰³⁴ (The idea has already been used in science fiction.²⁰⁷¹)

Dr. Conway Snyder at the Jet Propulsion Laboratory in Pasadena, California has performed a numerical simulation of the light-bending phenomenon at the Cytherian surface.²⁰⁶⁶ Let us imagine with him, for a moment, that we are aliens on the surface of Venus. Our eyeballs can see into the microwave region of the spectrum as well as the visible. What do we see?

The horizon appears to be elevated upward, all around us, at 9.4° from the horizontal. (Only 5°, if visibility drops to 200 kilometers.) Since Venus rotates backwards, the sun rises in the west and sets in the east, creeping across the sky at an imperceptible eight minutes of arc per hour. We are standing at the equator at the time of the equinox, so Sol lies directly over head at noon, Cytherian daylight time.

As the sun slowly falls toward the horizon, its shape begins to change. Its vertical dimension commences to shrink, while the horizontal component remains unchanged. At 6 PM Cytherian time, Sol should just be setting — but it isn’t. Instead, it lies 10.4° up, but is squashed down to a quarter of its normal size. By 7 PM the squashing has become 250:1 compared with the horizontal dimension, and by 8 PM, 30,000:1.

Sometime close to 12 PM, the tiny solar sliver suddenly increases in length dramatically, and at the stroke of midnight wraps itself around the horizon in a pencil-thin ring of light. The line then breaks in the east, the sun begins to reassemble itself in the west, and sunrise begins.

If we are more than 3/8° away from the solar latitude, however, the ring of light will not appear. Instead, we see the compressed sun-image “crawl like a worm across the horizon during the night, from the point where it has set to the place where it is planning to rise.”²⁰⁸⁶

* Calculations indicate the effect would be rather small, though, perhaps a few degrees inclination at most.²⁰⁶⁸ The first pictures back from the two Russian Venera spacecraft that landed on Venus in 1975 showed no evidence of the fish bowl,^2034,2079 but since the maximum range in the photos was only a few hundred meters the issue remains unresolved.

Figure 5.9 Scattering of Light in Planetary Atmospheres Scattering of Light Extinction of Light Particles Responsible Scattering of light by airborne particulate matter1993,1994,1995 Full Size   The family of curves at right represent the relative amount of light scattered away — at each particular frequency of light — by a hypothetical cloud of perfectly spherical, transparent, uniform size droplets suspended in air. However, ideal Mie monodispersions rarely occur in nature. Clouds of particles are of various shapes, textures, sizes, colors and degrees of opacity. Consequently, this graph is an abstracted, idealized version of reality and should be interpreted in that spirit. The beginnings of Rayleigh scattering are seen in the visible portion of the spectrum, in the right-hand section of the graph, where the particle radius r falls below a tenth of a micron or so. The flat parts of the curves stretching horizontally to the left indicate uniform scattering of all frequencies of light. The wavy parts in the middle demonstrate the oscillatory nature of the preferential scattering by color. Extinction of light by passage through a cloud of particles1995 Full Size   The three curves at right are simply a slightly different way of looking at the data in the previous graph. Here, we compare the relative attenuation of blue, green, and red light as it passes through the same cloud of idealized haze/fog particles we considered before. Note that for small droplets in the air, blue is preferentially scattered giving a blue sky. At large particle sizes, no frequency is preferred and the sky washes out white. For intermediate scatterer radii, red and blue alternate in their supremacy. Note also that green, whenever it predominates, is accompanied by large amounts of the other two colors — leaving a largely white sky with per haps the faintest of greenish tinges. Particles responsible for natural atmospheric scattering1994   This table lists the approximate size (in microns) and concentration of typical scattering particles normally present in Earth’s air. Aitken nuclei are hygroscopic (water-absorbing) microscopic condensation nuclei, the result of chimney flue gases, tobacco smoke, sulfur dioxide industrial emissions, and a host of natural sources.

What about the color of alien skies? Must they always be blue?²⁰⁵⁹ Of course, ETs will probably have different physiological seeing equipment than ours, but we shall permit ourselves the minor anthropocentric convenience of viewing their world through human eyes.

Light that reaches our eyes from the sky is merely sunlight scattered by the atmosphere. Had the Earth no air, our sky would appear quite black. This explains where the light comes from, but not why it is blue.

In 1899, a famous Englishman by the name of Lord Rayleigh devised an explanation for the color of the sky (Figure 5.9). According to his mathematical theory, scattering from very small particles (such as air molecules) increases as the fourth power of wavelength.¹⁹⁹⁵ This means that blue light, which has a very small wavelength, is highly scattered, while red light, with a relatively long wavelength, is scattered much less — sixteen times less, in fact.^1990,1991 So the blue light is preferentially removed from sunbeams and spread out uniformly from horizon to horizon. A little red is also present, and some yellow and green too, but blue is clearly predominant.

We can correct the Rayleigh theory for differences in planetary surface pressure and temperature. It turns out that the amount of light scattered is directly proportional to the atmospheric pressure, and inversely proportional to the temperature.¹⁹⁹⁴ So if we double the pressure we double the amount of light scattered in all colors — and the sky gets brighter generally. Doubling the temperature has the opposite effect: the intensity of scattering is cut in half. On the surface of a high pressure planet like Venus, the effect would be rather extreme. All colors would be so strongly scattered that the sky be comes a dim, featureless white.²⁰⁵⁹

In a perfectly clear, Earthlike atmosphere, the sky would be a rich blue hue. But we observe it to be a hazy, lighter blue. Why?

The Rayleigh theory applies only to particles which are much smaller than the wavelength of light, say, less than 10-100 Angstroms.¹⁹⁹⁴ If the scatterers in the air are much larger than this (as with dust in the atmosphere), Rayleigh’s formulation breaks down and the vastly more complicated Mie theory must be used¹⁹⁹⁵ — the details of which are beyond the scope of this book.

Rayleigh’s theory tells us that particles smaller than about 0.1 micron will preferentially scatter blue light. The Mie theory explains the behavior of atmospheres containing particles larger than about 4 microns. Above this critical size all frequencies of light are equally scattered, and the result is a gray or white sky. (Since there is always plenty of particulate matter, water haze and industrial pollutants floating around in the air — perhaps 100-1000 kg over each square kilometer — the sky’s sharp natural blueness is washed out unless we move to higher altitudes.)

Between 0.1 and 4 microns, the Mie theory becomes especially complex.¹⁹⁹⁵ The selection by color oscillates, sometimes preferring to scatter more blue and sometimes more red.^1993,1995 This effect is extremely sensitive to particle size. A uniform haze of 0.4 micron particles would scatter more blue (blue sky), but a similar cloud of 0.6 micron particles would produce more red (red sky).^1993,1994

If this is true, why don’t we commonly see such vivid colors in natural Earthly hazes and fogs? The reason is the natural fogs and mists contain a mixture of all sizes of particles, from one to ten microns or larger.¹⁹⁹⁵ As a result, these interesting color effects are added together randomly and average themselves out to a bland whiteness — which we do observe. If some reasonable mechanism could be proposed to get particles of a single, specific size into the atmosphere (i.e., a “monodispersion”); quite beautiful red and blue sky colors would be possible.

Barring this fascinating alternative, as particles of increasing size are added to a “Rayleigh atmosphere” the sky color will appear to change from dark blue to powdered blue, to whitish blue, and finally to grayish white.

A third factor affects sky color besides Rayleigh and Mie scattering. The color of the particles themselves is very important. A red particle, for instance, absorbs all light but red — which it reflects. Thus, it appears red in color. A green particle tend: to absorb blue and red but reflect green. (Under red or blue light such a particle would look black, but in green light it looks green.) So an atmosphere heavily laden with, say, green dust particles should also take on a distinct greenish hue.

We are now in a position to understand why the sky of Mars is red.^1989,2035 We add up the contributions from three effects: (1) Rayleigh scattering should give blue sky light, but will only be about 1% of its intensity on Earth because the Martian air pressure is only 0.01 atm;²⁰³⁵ (2) Dust motes an estimated two microns in diameter¹⁹⁸⁹ should produce a bright haze without color by Mie scattering; and (3) Particles in the Martian atmosphere are reddish surface dust, which reflect red light while preferentially absorbing blue and green. Hence, the sky of Mars is unusually bright, and appears a hazy “salmon pink” or “orange cream”¹⁹⁸⁹ (“embarrassed brick”?²⁰³⁵). It is clear that many other sky colors are similarly possible, provided a planet can be found with fine surface dust of the desired color.

There are other ways to get non-blue skies. For instance, we have discussed the process of frequency-selective light absorption by dust particles. Molecules of gas exhibit this property too.^619,620 The sky would no longer be blue under a fluorine atmosphere, to take one example. This gas absorbs blue strongly, and appears pale yellow in color. The sky would take on this color.

Chlorine air should appear green, because it absorbs light preferentially at the blue and orange-red ends of the spectrum. Similarly, an atmosphere of nitrogen dioxide would provide an orange-brown sky. If sulfur vapor is available, the air would alter color dramatically with large temperature changes. Near the boiling point at 720 K the sulfur sky would be dark yellow; as the temperature climbed to 770 K the atmosphere would turn a deep red, returning to straw yellow at about 1120 K.

The problem with using gases such as these is that they absorb light too darned well! At one atm pressure, a few meters of pure chlorine gas would transmit no visible light.²⁰⁵⁹ This is because even though blue and red are removed preferentially, some green is also eliminated. The sulfur vapor fares no better, sadly. At 1 atm pressure, blue light is cut to below human eye visibility in less than half a meter, and the red is gone in fifty meters.

So if the partial pressures of any of the aforementioned gaseous absorbers exceeds perhaps 0.001-0.01 atm, no light of any color will be able to reach the surface of the planet from the outside. Any inhabitants there must find their way around without the assistance of eyeballs.

If we want to use gaseous absorbers, it is better to choose weak absorbers instead of strong ones. For instance, under a deep ozone atmosphere the sky would probably appear reddish, because the gas is known to slightly absorb blue, yellow, and green sunlight rather well. Methane and ammonia, weak absorbers as they are, would provide a lovely blue-green sky (because absorption is mainly in the red) assuming the atmosphere was thick enough.²⁰⁵⁹

If the temperature at the surface is sufficiently high, another factor must be taken into account: blackbody radiation. Just as a stove’s heating element glows red when it is hot, so will the surface of a fried world like Venus. On Venus, red light emitted by the hot rocks could be orders of magnitude brighter than terrestrial moonlight — about like Earth on a dark, rainy day. In the blue the intensity would be about 100,000 times less than in the red, roughly 10% as bright as moonlight. Since red clearly predominates, reflections off the cloud base will give the appearance of a red sky, assuming fair or good visibility.

Still another trick to get colorful skies is to arrange for permanent colored cloud covers. Arthur Clarke suggests in Imperial Earth that the skies of Titan may be white with beautiful orange and red streaks and whorls, because of the presence of hydrocarbons and other organics in the atmosphere.¹⁹⁴⁷ This is similar to what is believed to impart coloration both to the orange bands and the Red Spot of Jupiter. Unfortunately, 20th-century humans are unlikely to find photochemical smog a very attractive method of obtaining unusual sky colors.

More aesthetically appealing are the possibilities of continuous luminescence, phosphorescence, and fluorescence as an adjunct to sky color phenomena.¹⁹⁹¹ But perhaps the most intriguing of all is the striking sunset effect called the “green flash,” which occurs just after the sun has dropped below the horizon.¹⁹⁹² The red and yellow light is not refracted enough to reach the observer at this point, and the blue has all been scattered away. This leaves only green, which is experienced as a brilliant flash during optimal viewing conditions.²⁰⁵⁹

But flashes on other planets could appear vastly different. Even on Earth, blue and violet flashes have been seen at higher altitudes.¹⁹⁹² On low-pressure worlds, where blue is scattered less (as on Mars), blue flashes may be the rule. the planet's rotation is slow enough, the “flash” could become a “glow,” lasting for seconds or even minutes.

Table 5.11 Rayleigh Molecular Scattering in Planetary Atmospheres as a Function of Stellar Class

It might be supposed that by changing stars one might be able to affect the color of the sky. After all, sky light is just scattered sunlight, and a class K sun puts out a lot more red than a class F star. However, as we see in Table 5.11, the consequences of illuminating an Earthlike atmosphere under the light of different stars are not great. Blue will predominate in the Rayleigh sky color, even if light from the coolest, reddest class M sun is used. On the other hand, we note that a terrestrial planet circling an F5 star will have skies of much deeper blue than a world associated with, say, a K2 sun. Stellar class is at best a very fine adjustment to sky color, in capable of countermanding the dictates of the atmosphere.

What about the appearance of the primary itself, as viewed from the planet’s surface? If the planet is in orbit around an orange or red star, the sun would seem bigger and redder than Sol does in our sky. Colors at the surface, illuminated by sunlight, would appear slightly different — the blues darker and the reds brighter. Shadows would have blurrier outlines than those on Earth. But an F5 star might cast sharper shadows, with a slight bluish tinge.⁸⁷⁷

As far as color is concerned, if the observers are beneath an atmosphere which either scatters the blue (blue sky) or absorbs blue preferentially (red sky), then light from the star will lose blueness and appear redder.¹⁹⁹¹ This effect is most striking at sunset on Earth, when the blue in Sol's rays is so completely attenuated that fiery red alone remains.¹⁹⁹⁰ Were the surface pressure perhaps five or ten times greater, Sol would appear similarly reddish at high noon and deep crimson at sunset (but much dimmer). Wispy puffs of clouds would catch the ruddy solar rays throughout the day, streaking and mottling the luminous azure sky with magnificent ever-changing patterns of coralline and cerise.

If the observers are at the bottom of an atmosphere which absorbs the red (blue sky) or scatters the red preferentially (red sky), the sun will appear bluer than normal.¹⁹⁹³ This effect has been seen, albeit rather infrequently, on Earth from time to time. Owing to the presence of particles at high altitudes following the great volcanic eruption at Krakatoa in August, 1883, the Moon took on a distinct blue-green color. This phenomenon of “blue moon” was observed in Great Britain on September 26, 1950, due to widespread fires covering a quarter-million acres of forestland in northern British Columbia, and on other occasions elsewhere.³⁶⁰ Blue suns and green suns are also possible in the same manner,¹⁹⁹³ and have been observed infrequently.²⁰⁷⁷

While the skies and seas of alien worlds are fascinating subjects for discussion, it is mainly upon the surface of a planet (its crust, or lithosphere) that life evolves and flourishes. Scientists who study mountain-building (orogeny), tectonic and seismic activity, and the construction of worlds generally, call themselves “astrogeologists” or “astrogeophysicists.”^598,2144

As Dole has pointed out, our knowledge of the forces responsible for earthquakes, volcanoes, and mountain-building is still incomplete.²¹⁴ One suggestion is that quakes and volcanoes are more likely on planets with higher gravitational compression and more internal heat generation due to radioactive decay. Planets smaller than Earth would tend to have less gravitational contractive force, relatively larger surface areas (compared to total mass) across which to radiate heat off to space,¹²³⁷ and relatively smaller volumes of heat-producing radioactive substances. Small worlds will thus tend to have lower internal temperatures,¹²³⁷ thicker and more solid crusts, and therefore much less volcanism and seismic activity.

Larger planets have relatively great volumes of radioactive material, higher gravitational compressive energy, and comparatively smaller surface-to-volume ratios (so it’s harder to get rid of heat).¹²³⁷ They should have larger molten cores, mantles that rise closer to the surface, and thinner crusts that can buckle and slip around more easily. If these suppositions are true in general for high-mass terrestrial worlds, more frequent and more severe quakes might be predicted, as well as higher levels of volcanic activity.

This theory squares with the reported characteristics of planets in our own solar system. The lightest world that has been intensively investigated is the Moon, within which only the faintest tremors have been detected deep below the surface.²⁰⁵⁶ The lunar lithosphere has solidified down to a depth of roughly 1000 kilometers.^1291,2043 When the core loses heat and contracts, the mantle is so thick and rigid it cannot buckle. Consequently, there is no real geologic surface activity on the Moon.^1291,2043

Mercury, the next most massive world examined by astrogeologists, is believed to have no surface tectonic activity at this time — although various surface migrations and volcanism a few eons ago are evident.^1565,2040 Mars apparently has seismic activity. The red planet also seems to have some lithospheric collapse due to mantle contraction, but there is no clear and convincing evidence for horizontal plate movements across the surface. It has been suggested that on Mars we may be seeing “incipient plate tectonics . . . where one plate is beginning to break away . . . like the Earth, about two hundred million years ago.”⁵⁹⁸ The towering Olympus Mons (formerly “Nix Olympica”¹³²³), at 26 kilometers high the largest mountain in the solar system, bears mute testimony to the presence of extensive and fairly recent volcanism on Mars.²⁰⁷²

Earth has well-developed tectonic activity, plenty of active volcanoes, and a crust only about 30 kilometers thick.³⁶⁷ Radar probes of Venus, our sister world, have found low mountain chains suggestive of at least a moderately active lithospheric environment.^1214,2041

Presumably, the core of a still larger terrestrial planet would be more massive and hotter, pushing the mantle closer to the surface. The thinner crustal sheet would buckle, slip and shake far more readily than does Earth’s rocky skin. Quakes would probably be more violent and more numerous, and breakthroughs in the crust by hot magma (volcanoes) should be widespread and commonplace.

What kinds of mountains are alien worlds likely to possess? The building of mountains is an extremely complex process, depending on planetary mass, gravity, composition, heat flow rate through minerals, air pressure and wind velocity, and a host of other factors. For instance, on larger worlds rivers may flow downhill faster because of the higher gravity, which may cut deeper valleys and canyons.

Perhaps one of the most significant astrogeological advances in this century has been the development and elaboration of the theory of continental drift. Continents are now known to be small plateaus of granite embedded in much larger “tectonic plates.” The entire Earth’s crust is believed to be fragmented into a mosaic of perhaps eight of these plates, rigid shifting masses of solidified lithosphere which have been described as great tabular “icebergs” of rock floating on the surface of a “sea” of denser subjacent mantle material.^2140,2141

Plates are believed to be about 100 kilometers thick,²¹⁴⁰ and may move literally thousands of kilometers across the surface of the planet in only 100 million years or so.²¹⁴² Convection currents in the deep mantle have been proposed as the prime mover of the plates, circulating the viscous magma in localized “cells” much like the currents of water in a flat pan which is heated from below.²¹⁴¹

Because the continents are always on the move (though they change shape very little as they travel piggyback around the world²¹⁴²), each has a trailing edge and a leading edge. The trailing edge is tectonically stable, so mountain-building is minimal. But the leading edge is forced downward with the descending mantle currents; the lighter, more siliceous materials that comprise the continents pile up at the site of subduction.²¹⁴¹ Great mountains are born. (One of the clearest examples of this process occurred during the Cenozoic Period, when the Indian Plate smashed into and dove under the Eurasian Plate, throwing up the mammoth Himalayan ranges.²¹⁴⁰)

From the arguments presented earlier, it is at least plausible to advance the hypothesis that more massive planets will have more internal energy available to drive the thermal convection currents in the mantle, and should therefore produce greater tectonic thrusting and more extensive mountain chains.

Figure 5.10 Maximum Size for a Planet's Mountains1279 Full Size

The graph at right gives the “maximum statically loaded topography” supportable by a range of different materials. The curves are based on the assumption that if the interior pressure created by building the mountain exceeds the compressive strength of the materials, then the mountain will “fall down.” Planetary radius R is the horizontal axis, and h, the maximum height of mountains (or depth of depressions), is the vertical axis, both in kilometers. For weaker materials — such as water-ice — the topographic relief must be far less than if rock is used. No materials are expected to have much greater strength than taenite, so all planets should be found below this line. (Note the extreme position of Jinx, a hypothetical egg-shaped planet devised by science fiction writer Larry Niven.451) Note the relative weakness of the ices — if Titan has only ammonia-ice mountains, they cannot be larger than two or three kilometers. Maximum mountain heights in our solar system are roughly as follows: Mercury — 3 km,1563 Venus — from 1-2 km,2041 Earth — from 8-11 km Luna — highest peak is 6.8 km high (Theophilus) Mars — highest peak is 26 km high (the volcano Olympus Mons)2072

Like all material bodies, mountains are subject to the Square-Cube Law. This principle is, quite simply, that volume increases faster than area as size increases. For a mountain to remain standing and not collapse, it must be strong enough to support its own weight. This weight is distributed over an area. The weight that must be supported, however, increases with the volume. (For example, mountains with eight times more mass have only about four times more base area to support that mass.) Consequently, a mountain should be less capable of sustaining its own bulk as it increases in size.

The maximum height of rocky ranges is therefore proportional to their weight, the product of the mass and the force of gravity (Figure 5.10). Higher gravity planets will have smaller, squatter mountains, because the limits of compressive strength of rock are reached much sooner. At least down to about 0.1 M_earth or so, smaller worlds should tend to have taller formations.

As has been discovered with craters on the bodies in our solar system,¹²⁷⁷the height of mountains should statistically vary inversely as the force of surface gravity.*

* Astrogeologists will recognize that I have made a gross oversimplification here. The mountains of large differentiated planets are actually supported by isostatic forces. Only small bodies can accurately be considered to have statically loaded topography.¹²⁷⁹

Table 5.12 Densities and Compression Strengths 1279,1569,1851, 1853,1854,1855

Mountain size will also be related to the compressive and shear strength of the building materials used.^1233,1279 The maximum height of ranges will vary approximately linearly with the compression strength (Table 5.12). For Earth mountains, rock is the usual orogen* with a maximum sustainable load of about 10⁷ kilograms/meter². However, were we to find mountains of carbon dioxide on another planet, the greatest height would be far lower. This is because the compressive strength of “dry ice” is less than 10-30% that of rock.¹⁵⁶⁹

Volcanism could be a peculiar affair on other worlds. On a planet as cold as Titan, for instance, water could be an orogen instead of a thalassogen. If sufficient crustal radioactivity exists, and if the planet is roughly terrestrial-sized, we might observe cold volcanoes spewing forth molten water instead of lava.¹⁹⁴⁷ Dr. Donald M. Hunten, a physicist at the Kitt Peak National Observatory, believes that Titan may possess just such a subsurface magma of liquid water.²⁰⁴⁶ The magma would lie atop a rocky mantle and would contain large amounts of dissolved ammonia. The relatively thin crust should then be a mixture of methane and water-ice, frozen solid.

A curious phenomenon is the flowing of glaciers (mountains of water-ice). There is some evidence that this may be virtually a unique property of H₂O “mountains,” One of the more unusual characteristics of water is its ability to drop its melting point when subjected to pressure. Underneath a glacier pressures rise to hundreds of atmospheres. A lubricating layer of melted ice can form at the base, and the object proceeds to slide downhill on this thin, slippery film of water.

While ice exhibits the freezing point depression effect up to pressures of more than 2500 atm, solid carbon dioxide and other ices cannot duplicate this behavior. Only water-ice will flow rapidly down valleys like rivers. One Alpine formation, the Quarayaq Glacier, is known to flow between 20 and 24 meters per day.¹⁸⁵⁰ (Of course, CO₂ glaciers are still subject to slow creep,¹⁵⁶⁹ but this is far less dramatic.)

If mountains are subject to the Square-Cube Law, are not worlds as well? Small, mountain-sized hunks of matter may be very irregular in shape, because the internal stresses are relatively low. But as mass increases, pressures build: Inside any terrestrial planet rock begins to flow and seek a spherical shape — energetically the most stable configuration.

* Derived from the Greek roots, meaning, literally, “something that produces mountains.” I use the word to signify “any substance capable of forming planetary mountains.”

Table 5.13 Maximum Size of Oblong (e = ½) Bodies, for Various Orogens1279

Stephen Dole has estimated that the largest mass of a body that can maintain a highly irregular shape is on the order of 10^-5 to 10^-4 M_earth.²¹⁴ To get some idea of the degree to which an object may deviate from sphericity, Table 5.13 gives the largest size of a body whose mountains are as tall as the planetary radius itself (the long axis is twice the short). These worlds must be very small to retain their egg-shape.

Finally, returning once again to peculiar surface effects, the astrogeologists may have some real surprises in store for us on other worlds. For example, we know that Venus’ air is deficient in oxygen, and one explanation is that the surface rocks have all been well-oxidized. But at temperatures beyond 620 K and pressures above 50 atm, superheated steam dissolves alumino-silicate rocks. If the oxygen depletion theory is correct, Venus might once have been molten to considerable depths and served as a factory for huge, exquisite gemstones.¹²⁹³ The surface of the Morning Star may well be studded with garnets, sapphires, rubies and topaz!